The present invention relates to a multiplexer with transmission line back-up function and, more particularly, to a multiplexer, which uses as its transmission line, i.e., as normal transmission line, a leased line and also use as a back-up line a parallel connection of a plurality of switched lines.
In prior art multiplexer of this type, high speed digital leased lines of high speed digital lines for internal communication in projects, have been used as transmission line. To enhance the reliability of the line, a back-up line is usually provided. Sometimes, a separate leased line is employed as the back-up line. However, it is uneconomical to employ the high fee leased line for the back-up purpose. Recently, therefore, use is made of economical ISDN switched line services. For the ISDN services, several interface menus are prepared. Among these interface menus, the highest speed one is for PCM primary speed group interfaces (called "ISDN Network 1500"). Thus, where a 6 Mbps band high speed digital leased line is used as the normal transmission line for multiplexed signal, for lines of the 1.5 Mbps "ISDN network 1500" are used for the back-up.
A prior art multiplexer system is shown in the block diagram of FIG. 4. Each multiplexer 5 comprises four 1.5 Mbps multiplexer units 51 to 54 for multiplexing and de-multiplexing twenty four 64 kbps channels to and from 1.5 Mbps band, and a 6 Mbps multiplexer unit 50 for multiplexing and de-multiplexing four primary group multiplexed signals from the 1.5 Mbps multiplexer units 51 to 54 to and from 6 Mbps secondary group multiplexed signals. Stations A and B each with the above multiplexer 5 installed therein, are connected to each other by a 6 Mbps high speed digital leased line 3 as the normal transmission line or by an ISDN switched network 4 as back-up line, the ISDN network 4 being constituted by four 1.5 Mbps switched lines 41 to 44 in parallel connection.
The leased and switched lines are four-line bi-directional lines. To the channel side of each multiplexer 5 are connected at most ninety six terminals 101 to 196. By the term "terminal" used in this specification is meant a terminal in a wise sense, covering various data terminals, host computers, FAX sets, telephone sets, video terminals, etc.
In the instant system, the 6 Mbps high speed digital leased line as the normal transmission line is normally used for communication via the 6 Mbps multiplexers 50. However, when the 6 Mbps multiplexer 5 in, for instance, the station A detects a trouble in signal received from the 6 Mbps high speed digital leased line 3, a leased line trouble signal (not shown) is issued to the 1.5 Mbps multiplexer units 51 to 54. In response to this signal, each 1.5 Mbps multiplexer unit sends a calling signal (not shown) to the side of the ISDN switched network 4 for line connection to the side of the station B.
Thus, the transmission line is switched to the side of the 1.5 Mbps switched lines 41 to 44 for back-up in primary group units of 1.5 Mbps. Meanwhile, in the station B the individual 1.5 Mbps multiplexer units 51 to 54 receive a called signal (not shown) accompanying the line connection from the ISDN switched network 4, whereby the transmission line is switched over to the 1.5 Mbps switched line side for back-up. In the above way, the back-up switching of the transmission line is realized.
In this prior art example, as described above, the multiplexing is performed first by the 1.5 Mbps multiplexer units for connection to the switched network for the back-up line and then by the 6 Mbps multiplexer unit for connection to the 6 Mbps leased line as the normal transmission line. That is, the multiplexing is performed in two stages, thus leading to high system equipment cost.
In addition, depending on the traffic, there are unnecessary channels for any back-up so that it is desired to reduce the 1.5 Mbps lines to three lines. In this case, it is necessary to switch channels which need back-up and those which do not, which leads to the necessity of changing the wiring from the terminal. Further, if it is desired to replace the 1.5 Mbps switched lines with 384 kbps switched lines as the back-up line, it is necessary to change the system equipment, thus giving rise to extra expenses. That is, such back-up line change dictates hardware change, requiring man-hour and extra expenses. In other words, the coping property in such case is unsatisfactory. Further, in recent multiple media multiplexers it is impossible to cope with such cases as when it is to provide the back-up in units of 6 Mbps multiplexed signals.